. . . prepare for anything. We've been and it's been chilly (nothing like up North, but still chilly for what you might expect) and we've been that it's been hot. Chances are shorts will be in order during the day, but you'll want jeans and a sweatshirt or something at night when it can get cooler.
Have a blast! It will be wonderful.
For your first time, the parks will be your main focus and you won't need much more, but there are lots of other things to do so check out the sites mentioned before.
A sampling of optional activities could include: renting water mice, visiting DisneyQuest, shopping in DownTown Disney, going to a water park, Circque Du Soleil etc.
Just enjoy!